In integrated circuit devices, it is often desirable to be able to permanently store information, or to form permanent connections on the integrated circuit after it is manufactured. Fuses or other devices forming fusible links are frequently used for this purpose. A fused based cell can permanently store data, for example, where a burned fuse represents a “1” and an unburned fuse represents a “0.” Similarly, fuses can form permanent connections on integrated circuits after they are manufactured. Where fuses are used for information storage purposes, a sensing circuit is typically used to determine the state of a fuse (whether the fuse has been programmed or not). The sensing circuit is used to determine the value that is stored. Sensing circuits operate to distinguish between programmed and unprogrammed (or burned and unburned) fuses, usually by detecting a change in the resistance of the fuse device from a low to a high value.
Advances in semiconductor manufacturing technology have decreased the resistance of fuses after they have been burned, referred to as post-burn resistance. Similarly, the operating voltages of most semiconductor devices have been reduced in pursuit of lower power consumption. While higher burning voltages move the post-burn resistance to a high level, the higher voltages may damage semiconductor devices designed to operate at lower voltages. It would be useful to have the ability to use higher burning voltages for fuse-based devices, while providing protection for devices that operate at lower voltages.
As semiconductor manufacturing processes move to lower supply voltages, the voltage available to program fuses is decreased. As the fuse programming voltage is lowered, the number of “marginally burned” fuses increases. Fuses are considered to be marginally burned when, after programming, the resistance of the fuse remains low enough that there is an unacceptable risk that the fuse might be identified as being unprogrammed when its state is sensed. Therefore, marginally burned fuses may compromise the functionality of quality of circuit that uses the fuse. This is particularly true where the state of the single fuse determines that state of a fuse based storage cell. Additional or redundant fuses have previously been provided for this type of cell, but each redundant fuse takes up valuable space.
Some fuse based cells are designed to be programmed using a higher programming voltage than the core supply voltage used in the normal system operation. The use of a higher programming voltage enables reliable programming of fuses by insuring that the number of marginally burned fuses is minimized. One such fuse based cell uses vertical diffusion metal oxide semiconductor (VDMOS) transistors to protect the other transistors in the cell from damage during programming events during which the programming voltage is raised substantially above the supply voltage. The VDMOS transistors can withstand higher voltages without degradation than the non-VDMOS transistors. While the VDMOS transistors are able to withstand high voltages, they unfortunately have higher output resistance than regular transistors. In addition, compared with regular complementary metal oxide semiconductor (CMOS) transistors, VDMOS transistors tend to exhibit more variation in their characteristics because they are an auxilliary device and are not the focus of the same kinds of process developments as seen with process developments for CPU performance.